1. Field of the Invention
The present invention relates to a method for cooking a lignocellulose material, particularly to an effective cooking method for pulp, wherein a polysulfide cooking liquor and a quinone compound are used in combination.
2. Discussion of Background
The principal method for producing chemical pulp which has heretofore been industrially employed, is an alkaline cooking method of a lignocellulose material such as wood chip, whereby a kraft method employing an alkaline cooking liquor comprising sodium hydroxide and sodium sulfide as the main components, has been used in many cases. Further, as one of cooking methods to improve the yield of pulp, a so-called polysulfide cooking method is widely known, wherein cooking is carried out by means of an alkaline cooking liquor containing polysulfides. According to this polysulfide cooking method, polysulfide ions oxidize and stabilize terminal aldehyde groups of cellulose and hemi-cellulose, to prevent a peeling reaction and to suppress a reaction for elution of cellulose and hemi-cellulose, whereby the yield of pulp will be improved. And, in general, the higher the concentration of the polysulfide sulfur in this polysulfide cooking liquor, the higher the cooking effects.
The alkaline cooking liquor containing polysulfides, to be used in the above cooking method, is produced by a method of air oxidation in the presence of a catalyst (for example, JP-B-50-40395, JP-A-61-257238, JP-A-61-259754, JP-A-09-87987). In this method, when usual white liquor is employed, it is possible to obtain an alkaline cooking liquor having a polysulfide sulfur concentration of about 5 g/l (l represents litter, the same applies in this specification) at a reaction rate of about 60% and a selectivity of about 60%. However, during the formation of polysulfides, this method produces thiosulfate ions as a by-product which is ineffective for cooking, whereby it has been difficult to produce an alkaline cooking liquor containing highly concentrated polysulfide sulfur at a high selectivity.
On the other hand, as shown in e.g. JP-B-57-19239, JP-B-53-45404 and JP-A-52-37803, a quinone cooking method is also widely known, wherein cooking is carried out by adding a quinone-hydroquinone compound to an alkaline cooking liquor. The added quinone compound oxidizes and stabilizes the terminal aldehyde groups of cellulose and hemi-cellulose, thereby to prevent a peeling reaction and suppress an elution reaction of cellulose and hemi-cellulose. On the other hand, the quinone compound which has become a hydroquinone type, will act on lignin to reduce and elute the lignin and to become a quinone type itself. Thus, the quinone-hydroquinone compound stabilizes cellulose and hemi-cellulose and accelerates delignification by the oxidation-reduction cycle of itself, whereby even when compared under such a condition that the Kappa number of pulp is the same, it brings about effects to improve the yield and at the same time to reduce the amount of active alkali required for cooking. Here, in this specification, the quinone-hydroquinone compound means both a quinone compound as an oxidation type quinone substance and a hydroquinone compound as a reduction type hydroquinone substance.
In the Journal of Japan Technical Association of Pulp and Paper Industry, Vol. 32, No. 12, p. 713–721 (1978), Nomura et al. disclose that in cooking for kraft pulp employing a cooking liquor comprising sodium hydroxide and sodium sulfide as the main components, which is commonly adopted as a cooking method for pulp, if a quinone compound is employed, of which the oxidation-reduction potential in the form present during the cooking, which potential is a value calculated as a standard oxidation-reduction potential (Ea) with a hydrogen ion activity of 1, is from 0.1 to 0.25V to the standard hydrogen electrode potential, it is possible to improve the yield, etc. of pulp, and they disclose that even within this potential range, a quinone compound such as anthraquinone carboxylic acid or anthraquinone dicarboxylic acid having a potential higher than 9,10-anthraquinone (Ea=0.154V) is inferior in the effects, and a quinone compound such as hydroxyanthraquinone having a low potential has larger effects than 9,10-anthraquinone.
Further, as shown in e.g. JP-A-7-189153, a so-called polysulfide-quinone cooking method having the above-mentioned cooking method combined, is also widely known. By this cooking method, the above-described effects appear synergistically. Namely, as effects of the polysulfide-quinone cooking, improvement in the yield of pulp as compared with the same Kappa number and reduction in the amount of active alkali to be used as compared with the same amount of pulp production, can be accomplished over the cases where the respective techniques are separately employed.
However, no research or development has been made on what type of quinone compounds is effective for cooking and for improvement in the yield of pulp or in the required amount of the chemical solutions to be used, in the presence of polysulfides. In the present invention, a research and study have been made on a cooking method relating to such an aspect, and as a result, it has been found that further improvement in the yield of pulp, further reduction in the amount of the chemical solutions to be used, and solution of the problem relating to the load on the recovery boiler, can be accomplished, thus arriving at the present invention.